Standpipe

Newbie here so apologies if I am posting in the wrong section. In the last two weeks in our downstairs bathroom we have been encountering 5-15 tiny black flies. Very puzzled as to where they are coming from.

My husband has thoroughly cleaned out the whole room, even taking the trap off the sink in case they were breeding in there.

As a final resort on Sunday he took the cover off the washing machine and cleaned the trap out on that and ran vineger through the machine. Took the hose out of the standpipe and poured some enzyme based cleaner in the hose and the standpipe. Covered it with tape for three days and no bugs.

Today we took the tape off and ran two loads of laundry. Low and behold tonigjt we are encountered with bugs again.

Is it possible bugs are breeding in the machine or in the standpipe. I would not think so given the water pressure flowing through there. If so is it possible to clean the standpipe out or pour drano down there to clean out.

At our wits end as to what to do here. Had thought about the pest person but worried they will spray which will clear it for a few days and they will be back again.

Drain flies get in when there is a trap with no water in it. Cleaning things is nice, but the drain flies will move about no matter if it is clean or not. Often the culprit is a fixture that is not often used which allows the water in the trap to evaporate.

The room only has a toilet, sink washer and dryer in it. There was goopy looking water in the trap which we vacuumed out this morning to see if something gross was in the trap. Nothing but some dirty water. The water did smell kind of gross. There is a bathroom above this room which our kids barely use. We just poured some hot water down there hj if that was what you were suggesting.

We did go away from the house for a week and this started happening after we came back.

No not a dry trap !
I used to have the same problem what you need to do is pour a big pot of boiling water down all your drains except toilets,
they will break so you use a couple of cubs of bleach into toilets to kill the pest eggs !
They is why they keep returning give it a week then treat again if they are still around